Post common envelope binaries from SDSS^*

IV. SDSS J121258.25–012310.1, a new eclipsing system

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: angm@aip.de
² Departamento de Fisica y Astronomia, Universidad de Valparaiso, Avenida Gran Bretana 1111, Valparaiso, Chile
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Instituto de Astrofísica de Canarias, Vía Láctea, s/n, La Laguna, 38205, Tenerife, Spain


Received: 28 October 2008
Accepted: 2 January 2009

Abstract

From optical photometry we show that SDSS J121258.25–012310.1 is a new eclipsing, post common-envelope binary with an orbital period of 8.06 h and an eclipse length of 23 min. We observed the object over 11 nights in different bands and determined the ephemeris of the eclipse to HJD, where numbers in parenthesis indicate the uncertainties in the last digit. The depth of the eclipse is mag in the V band,  mag in the R band and  mag in the I band. From spectroscopic observations we measured the semi-amplitude of the radial velocity  km s^-1 for the secondary star. The stellar and binary parameters of the system were constrained from a) fitting the SDSS composite spectrum of the binary, b) using a K-band luminosty-mass relation for the secondary star, and c) from detailed analyses of the eclipse light curve. The white dwarf has an effective temperature of  K, and its surface gravity is . We estimate that the spectral type of the red dwarf is and the distance to the system is  parsec. The mass of the secondary star is estimated to be in the range   , while the mass of the white dwarf is most likely  . From an empirical mass-radius relation we estimate the radius of the red dwarf to be in the range   , whereas we get    from a theoretical mass-radius realation. Finally we discuss the spectral energy distribution and the likely evolutionary state of SDSS1212–0123.